The invention relates generally to a current limiter and more particularly to a superconducting current limiter with a transformer coupled trigger mechanism.
Current limiting devices are critical in electric power transmission and distribution systems. For various reasons such as lightening strikes, short circuit conditions can develop in various sections of a power grid causing sharp surge in current. If this surge of current, which is often referred to as fault current, exceeds the protective capabilities of the switchgear equipment deployed throughout the grid system, it could cause catastrophic damage to the grid equipment and customer loads that are connected to the system.
Superconductors, especially high-temperature superconducting (HTS) materials, are well suited for use in a current limiting device because of their intrinsic properties that can be manipulated to achieve the effect of a “variable impedance” under certain operating conditions. A superconductor, when operated within a certain temperature and external magnetic field range (i.e., the “critical temperature” (Tc,) and “critical magnetic field” (Hc,) range), exhibits no electrical resistance if the current flowing through it is below a certain threshold (i.e., the “critical current level” (Jc,)), and is therefore said to be in a “superconducting state.”
However, if the current exceeds this critical current level the superconductor will undergo a transition from its superconducting state to a “normal resistive state.” This transition of a superconductor from a superconducting state to a normal resistive state is termed “quenching.” Quenching can occur if any one or any combination of the three factors, namely the operating temperature, external magnetic field or current level, exceeds their corresponding critical level.
The surface plot shown in FIG. 1 illustrates the inter-dependency among these three factors (Tc, Hc, and Jc,) for a typical superconducting material type. As shown in FIG. 1, the surface plot includes three axes T, H, and J, where Tc is the critical temperature below which the superconducting material must be cooled to remain in the superconducting state, where Hc is the critical magnetic field above which the superconducting material cannot be exposed in order to remain in a superconducting state, and where Jc is the critical current density in the superconducting material that cannot be exceeded for the superconductor to remain in a superconducting state. The “critical J-H-T surface” represents the outer boundary outside of which the material is not in a superconducting state. Consequently, the volume enclosed by the critical J-H-T surface represents the superconducting region for the superconducting material.
A superconductor, once quenched, can be brought back to its superconducting state by changing the operating environment to within the boundary of its critical current, critical temperature and critical magnetic field range, provided that no thermal or structural damage was done during the quenching of the superconductor. An HTS material can operate near the liquid nitrogen temperature 77 degrees Kelvin (77 K) as compared with a low-temperature superconducting (LTS) material that operates near liquid helium temperature (4 K). Manipulating properties of a HTS material is much easier because of its higher and broader operating temperature range.
The quenching of a superconductor to the normal resistive state and subsequent recovery to the superconducting state corresponds to a “variable impedance” effect. A superconducting device with such characteristics is ideal for a current limiting application. Such a device can be designed so that under normal operating conditions, the operating current level is always below the critical current level of the superconductors, therefore no power loss (I2R loss) will result during the process. When the fault conditions occurs the fault current level exceeds the critical current level of the superconducting device, thus creating a quenching condition. By the same token, mechanisms altering the device operating temperature and/or magnetic field level can be put in place either as a catalyst or an assistant to achieving faster quenching and recovery of such a superconducting device.
For some HTS materials such as the bulk BSCCO elements, there often exist, within the volume of the superconductor, non-uniform regions resulted from manufacturing process. Such non-uniformed regions can develop into the so-called “hot spots” during the surge of current that exceeds the critical current level of the superconductor. Essentially, at the initial stage of the quenching by the current, some regions of the superconductor volume become resistive before others do due to the non-uniformity. A resistive region will generate heat from its associated i2r loss. If the heat generated could not be propagated to its surrounding regions and environment quickly enough, the local heating will damage the superconductor and could lead to the breakdown (burn-out) of the entire superconductor element.
U.S. Patent Publication Ser. No. 2003/0021074A1, Ser. No. 10/051,671, published Jan. 30, 2003, entitled, “Matrix-type Superconducting Fault Current Limiter” assigned to the assignee of the present invention, incorporated by reference in its entirety, uses a mechanism that combines all three of the quenching factors of the superconductor, namely current, magnetic field and temperature, to achieve a more uniformed quenching of superconductor during current limiting. This so-called MFCL concept can dramatically reduce the burnout risks in bulk superconducting materials due to the non-uniformity existed in the superconductor volume. In addition, the detection of a fault and subsequent activation of the current-limiting impedance of the MFCL are done passively by built-in matrix design, without assistance of active control mechanism. This makes a fault current limiter based on the MFCL concept more easily designed, built and operated for a wide range of potential current-limiting applications.
The MFCL concept utilizes the voltage generated by the quenching of superconducting elements in the so-called trigger matrix and the magnetic field generated in parallel-connected trigger inductors ( ) from that voltage, to quench the superconducting elements in the so-called current-limiting matrix. The magnetic coupling is achieved by physically wind the parallel-connected coils of the trigger matrix, directly around the superconducting elements in the current-limiting matrix. Because of this intricate relationship between the elements of the two matrices, the design of the MFCL requires careful consideration of voltage, magnetic field strength, coil design and various other factors.